This application relates to hard disc drives and more particularly to an apparatus and method for exciting a servomechanism in a disc drive with a low frequency signal, without necessitating the inclusion of a model of the servomechanism""s response to the low frequency signal.
The storage medium for a disc drive is a flat, circular disc capable of retaining localized magnetic fields. The data, that are stored upon the disc, find physical representation through these localized magnetic fields. The data are arranged on the disc in concentric, circular paths known as tracks.
The localized magnetic fields can be detected by a head when the field is brought in close proximity to the head. During operation the disc continually rotates, meaning that for each rotation, a head fixed a given radius from the center of the disc would encounter every localized magnetic field along a given track. Altering the radial coordinate of the head allows the head to read or write data along a different track.
The head is mounted upon an actuator arm that is rotated by a servo control system. Accordingly, the track position of the head is controlled by the servo system. When the head needs to access a different track, the actuator arm is rotated, bringing the head to the desired track position. The process of moving the head to a new track, referred to as seeking, includes an acceleration and a deceleration phase, and the period during which seeking occurs is known as the access time.
In a conventional disc drive, a seek operation is controlled by a feedback loop, and may additionally use feedforward, which has the benefit of allowing the control system to operate with a lower bandwidth. In such a disc drive, the control process typically works as follows. During the acceleration phase of the seek operation, a feedforward signal is used to excite the servo system, resulting in the head being accelerated along a circular arc. While the head is experiencing acceleration, its velocity and position are periodically measured, and these measurements are compared against target velocity and position values. The differences between the measured values and the target values are subsequently used to adjust the feedforward signal that excites the servo system.
During deceleration, a negative feedforward signal is used to excite the servo system, resulting in the head being decelerated. Once again, while the head is decelerated, its velocity and position are periodically measured, and these measurements are compared against target velocity and position values, the differences therebetween being used to subsequently adjust the feedforward signal that excites the servo system.
One cost effective means of implementing the above described control system is to use a square wave as the feedforward signal (with the positive portion of the wave correlating with acceleration, and the negative portion with deceleration). The advantage of doing this is that the feedforward signal can be generated from a single table stored in a memory device, as opposed to requiring a different table for each length of seek to be performed. Likewise, a similarly shaped tripartite waveform can be used as a feedforward signal, wherein the constant positive and negative portions of the signal are separated by a period of quiescence of equal duration. Acceleration, in such a waveform, would correspond to the positive going portion of the wave, deceleration with the negative going portion, and the quiescent portion (no output) would correspond with a coast state. Again, this feedforward signal could be generated from a single table. Accordingly, such an approach requires less memory space and therefore reduces the total cost of the disc drive.
The cost effective implementation just described possesses a disadvantage: since the feedforward signal used to excite the servo system is a square wave, it therefore contains high frequency components. The high frequency components of the square wave cause the actuator arm assembly to resonate, potentially producing errors. The high frequency components of the square wave are also associated with acoustic noise, meaning that the disc drive will be noisy during operation, an undesirable characteristic from a consumer standpoint.
The method and apparatus in accordance with the present invention solves the aforementioned problems and other problems by allowing the servomechanism to be excited by a low frequency stimulus, yet requiring the storage of a model of only the servomechanism""s response to a high-frequency (square-wave) stimulus. The method involves filtering, with a first low-pass filter, a high frequency input signal, yielding a filtered input signal, which is used to excite the servomechanism. The various responses of the servomechanism are measured at intervals. The various responses to be measured may include head position or any number of its infinite derivatives (velocity, acceleration, etc.). Next, the various responses of the servomechanism to the unfiltered, high frequency input signal are modeled. The modeled responses of the servomechanism are then filtered with a low-pass filter, yielding filtered modeled responses. Finally, the filtered input signal is adjusted based upon the difference between the filtered modeled responses and the measured responses of the servomechanism.
The apparatus includes a first low-pass filter, for filtering a high frequency input signal, and yielding a filtered input signal. The filtered input signal is received by an adder (this being only one of its inputs), with its output being operably connected to a driver and a first input of a measuring module. The driver supplies current to the servomechanism, based upon the sum provided to it by the adder. The servomechanism receives the current, and being excited thereby is accelerated. This response is measured by a measuring module, which is coupled to the servomechanism. The apparatus also includes a servomechanism model which predicts the various responses of the servomechanism to having been excited by a high frequency stimulus (square-wave). The servomechanism model is connected to a low-pass filter module that attenuates the high frequency bands of the various modeled responses. An error signal-generating module receives the various filtered modeled responses as one input, and receives the various measured responses from the measuring module as another input, yielding an error output, which is provided as an input to a compensator. The compensator is designed to stabilize the control system, and has its output coupled, as an additional input, to the adder.